Whole parts of African city with time regressing (5)

I believe the answer is:

units

I'm a little stuck... Click here to teach me more about this clue!

'whole parts' is the definition.
(individual units)

I cannot understand how the rest of the clue works.

(Other definitions for units that I've seen before include "Measures of alcohol" , "Shops in a mall, for example" , "military teams" , "Apparatus similar to barbecue" , "Modules" .)

I've seen this clue in The Times.

Want a hint initially instead of a full solution? Install my app